1. Field of the Invention
The present invention relates generally to the field of RF communication systems, and particularly to radar systems.
2. Technical Background
A radar is a system that uses electromagnetic waves to detect objects within a certain spatial volume. A radar system may be used to determine the range, altitude, direction, and/or speed of fixed objects, or objects that are moving through the spatial volume of interest. Radar systems have been used to identify and/or track various and disparate objects such as aircraft, ships, motor vehicles, weather formations, terrain and baseballs. The term “radar” is an acronym for “RAdio Detection And Ranging.” As the name implies, a radar system transmits radio waves into the spatial volume referred to above. If and when a radio wave comes in contact with an object in space, the radio wave will be reflected and scattered by the object. Thus, a reflected signal is propagated back toward the radar system. The radar system receives the reflected radio wave and detects the object. Of course, a radar system is typically configured to transmit many radio pulses into the spatial volume every second. Each of these pulses are received and detected by the radar system. By comparing these pulses the radar system can determine if the object is moving, and if moving, its speed and direction.
The above discussion implies that a radar system includes several major parts, i.e., a transmitter, a receiver, and some sort of processing capabilities. One can further imagine that the transmitter and receiver employ an antenna to transmit the radio waves into space or to capture the radio waves propagating in space. The receiver is typically, but not always, disposed in the same location as the transmitter. A received reflected radar signal is usually very weak (indicating that the object is either small, a great distance from the receiver, or both) and must be amplified before it is processed. A radar system, therefore, is well suited to detect objects at great distances from the radar system and is useful in military, air traffic control, meteorology, automotive traffic control (i.e., speeding), etc.
In certain prior art radar systems, the antenna is rotated mechanically. The beam radiated by the antenna is propagated into space along the bore sight of the antenna. The spatial volume is, therefore, scanned by rotating the antenna, typically in a 360° sweep. One of the drawbacks of this approach relates to the cost and reliability of the mechanical equipment used to rotate the antenna. When ever a system uses moving parts, the system will ultimately wear out and break. Thus, the system must be maintained and replaced over time. The drawbacks associated with mechanically rotated radar antennas can be substantially obviated by the use of a phased array antenna.
A phased array radar antenna includes a plurality of antenna elements disposed in a two-dimensional array. These antenna elements are used for both transmission and detection of electromagnetic energy in an alternating fashion. A phased array radar system does not require moving parts, but may have them. For example, a planar array may be rotated mechanically to cover a required azimuthal range. However, a phased array radar does not require mechanical steering; it can be steered through phase shifting, or time delaying, signals to the various elements. In any event, a phased array radar beam is emitted by the plurality of elements using a principle known as superposition whereby the radio waves emitted by each element in the phase array are combined. Moreover, the amplitudes and phases of the radio waves constructively and destructively interfere with each other to create a composite radar beam having a predetermined radiation pattern. By continuously varying the amplitudes and phases of the radio waves being emitted from the various elements of the array, the composite radar beam may be pointed in a certain direction, or be made to scan back and forth (i.e., in azimuth) or up and down (i.e., in elevation). Thus, a phased array antenna propagates a single beam into the spatial volume and the reflected return signals are received by all of the elements in the phased array. Accordingly, a phased array radar system may be viewed as a single-input multiple output (SIMO) system because the antenna array transmits a single composite radar beam and the reflected signal is received by all of the elements in the phase array.
Recently, a multiple-input multiple-output (MIMO) radar architecture has been proposed. As its name suggests, a MIMO architecture employs multiple independent transmitters (i.e., inputs) and multiple receivers that are configured to take advantage of the geometry of the transmit and receive locations to increase target resolution. In some MIMO architectures, each transmitter may employ an omni-directional antenna having a gain equal to one (1). Since the transmitter is omni-directional with little gain, achieving a desired signal to noise ratio (SNR) on a given target requires a longer integration time, resulting in enhanced Doppler resolution. Further, if the multiple transmitter elements are to radiate at the same time, the set of transmit waveforms must be comprised of orthogonal waveforms. Time or frequency orthogonality are other ways to achieve orthogonality. From a mathematical perspective, two signals are “orthogonal” if their “dot product” is equal to zero. From a certain perspective, therefore, if two signals are orthogonal, it means that they are unrelated. Thus, a set of orthogonal signals includes signals that are unrelated to each other. Having each transmitter direct an orthogonal signal into the search volume allows each receiver to distinguish the transmission source of a received reflected signal. Thus, using a set of orthogonal signals is very useful. It should also be noted that the transmit antenna elements may have some gain or pattern in some MIMO schemes.
On the receive side, each receiver element may receive reflected signals generated by each transmitter and must be configured to accommodate each orthogonal signal. Each receiver channel, therefore, must include a matched filter for each orthogonal signal. A matched filter performs a mathematical function known as a cross-correlation whereby the received signal is convolved with one of the known orthogonal signals. If the received signal includes a version of the orthogonal signal (i.e., indicating that the received signal was generated by the orthogonal signal being reflected from a target in the spatial volume), the matched filter will indicate that it has detected a match between the transmitted signal and the received signal. As noted above, each receiver channel must include a matched filter for each orthogonal signal included in the set of transmitted orthogonal signals if it is to have the ability to detect them all.
To put it quite simply, the benefits and the drawbacks of the MIMO architecture are like two sides of the same coin. As noted above, the MIMO architecture individually filters and processes each received signal relative to each orthogonal signal prior to estimating the position of a target in the spatial volume. Because the MIMO architecture uses much more data in performing these calculations, it provides superior angle resolution and Doppler resolution vis a vis conventional coherent radar architectures. Superior angle resolution and Doppler resolution are typically touted as reasons why the MIMO architecture is superior to conventional coherent radar systems. The other side of the coin, i.e., the drawbacks associated with the MIMO architecture, relates to the increased processing requirements. In other words, implementing a MIMO system is challenging because of the intense processing requirements associated with MIMO architectures. The aforementioned processing requirements translate into cost, size, weight and power consumption realities that make MIMO architectures impractical to implement.
Moreover, providing extremely high transmit power from edge-located omni-directional transmit antennas in a volume search application is impractical. Omni-directional transmit antennas waste radar energy in portions of angle space that are not a part of the desired search volume. In addition, very long coherent integration times are not practical for the detection of a target with high Doppler shift due to range smearing during integration, unless range resolution is decreased significantly. This is not desirable for most volume search radars. As discussed above, a large MIMO array requires large numbers of simultaneous orthogonal transmit waveforms. An excessively large matched filter bank is required to process the waveforms and achieve a reasonable response from moving targets. Again, this requires a signal processor that has a very significant size, weight and cost, and one that consumes an inordinate amount of power.
What is needed, therefore, is a radar system that substantially eliminates the drawbacks associated with MIMO architectures while retaining the benefits. What is further needed is a radar system that is dynamically reconfigurable in real-time to search any desired volume. What is also needed is a radar system that incorporates MIMO features such that tradeoffs between detectability and accuracy are dynamically optimized in accordance with changing real-time mission requirements.